In an exhaust system of an internal combustion engine which performs lean combustion such as in a diesel engine, a NOx catalyst device is arranged to purify NOx in the exhaust gas. A known NOx catalyst device produces ammonia by hydrolysis of urea supplied with the exhaust gas and selectively reduces NOx in the exhaust gas by use of the produced ammonia NH3 as a reducing material.
To decrease ammonia which is not used to reduce NOx in such a selectively reduction type NOx catalyst device and which is released into the atmosphere, it has been suggested that an ammonia oxidation catalyst device be arranged downstream of the selectively reduction type NOx catalyst device.
However, in such an ammonia oxidation catalyst device, when ammonia is oxidized at a relatively low temperature, dinitrogen monoxide (or nitrous oxide) N2O is produced. It is undesirable to release N2O into the atmosphere. Accordingly, it has been suggested that the temperature of the ammonia oxidation catalyst device be elevated not so as to oxidize ammonia at a relatively low temperature (for example, refer to Japanese Unexamined Patent Publication No. 2008-255899).